1. Field of the Invention
The present invention relates to a method of and apparatus for measuring the density of fine particles suspended in a fluid such as oil, water or air. More particularly, the present invention pertains to a method of and apparatus for optically measuring the density of fine particles suspended in such fluid.
2. Description of the Prior Art
Measurement of the density of fine particles suspended in fluids (e.g., dust and bubbles in oil, solid particles in water, or dust in air) are often effected using optical means. In these conventional optical methods, a light source, a fluid which is an object of measurement and a photodetector are disposed on a straight line so that the light from the light source is applied to the fluid, and the light having passed through the object is detected by the photodetector. Thus, the density of fine particles suspended in the fluid is measured on the basis of the fact that the quantity of light detected by the photodetector is smaller than the quantity of light emitted from the light source due to absorption or scattering of light by the fine particles.
With the above-described prior art, however, when the size and density of fine particles are small, the difference between the quantity of light from the light source and the light quantity detected by the photodetector is correspondingly small. There is, therefore, a limit in the measurable size and density of fine particles, and it has therefore been impossible to measure extremely fine particles and the density thereof. In order to obtain a required degree of accuracy in measurement, it is necessary to increase the difference between the quantity of light from the light source and that detected by the photodetector, and this involves the necessity of increasing the volume of a sample fluid as an object of measurement. An increase in volume of the sample fluid in turn causes the size of the apparatus to increase as a whole.